I think the move was fine for this season, and things have worked out very well with PCA as it is, but there is a bigger problem next year if Bellinger doesn't opt out. The Cubs would enter the off-season with players essentially locked into all 9 spots in the lineup. In some ways, that's a good problem to have, but it doesn't leave much flexibility to improve the roster without significant trades. It also continues to block the remaining AAA outfielders for the foreseeable future.
While I think the Cubs would be in a better position headed into the off-season without Bellinger, I still think it was the right move to make at the time. The one thing the Cubs appear to have got extremely lucky with is the Bellinger injury. If he hadn't broken his finger, how much would PCA have played? Would he have been able to make progress with only limited at bats, or would he still be relegated to the role of light hitting pinch runner and defensive replacement? Sometimes you just need to let young players struggle while they figure things out, and the Cubs may have benefited from being forced into doing that.